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Back in the ancient past (1991), the MIDI Manufacturers Association (MMA) and the Japan MIDI Standards Committee (JMSC) got together and added a chapter to the standard called "General MIDI System Level 1." The basic idea was to provide multimedia developers and consumers with an easy way to ensure that when the composer specifies "piano on track 1" that what plays back on every GM system is, in fact, a piano sound on track 1, played at the same pitch as originally defined.
The GM standard calls for 24-voice polyphony dynamically allocated from a minimum of 16 MIDI channels. All voices must respond to velocity, pitch bend, and mod wheel information. The sound set includes at least 128 sounds divided into sixteen different families of instruments such as Piano, Brass, Synth, and Sound Effects. Each family consists of eight variations on the main theme. Piano, for example, includes Acoustic Grand, Bright Acoustic, Electric Grand, Honky-Tonk, Electric Piano 1, Electric Piano 2, Harpsichord, and Clavinet.
If the GM synth includes drum sounds, the GM standard addresses that as well. Each drum is assigned to a particular key, so, for example, when you play MIDI Note #36, you always get a kick drum. The MIDI channel for GM drums is always channel 10.
What the GM standard omits is how these requirements must be met. For example, the piano patch can be a beautifully recorded, layered, and multisampled extravaganza. Or it can be a low-bandwidth sound that only vaguely resembles the real thing. Another example is velocity - it can be implemented in such detail that any one of several layers can be addressed, or as rudimentary as adjusting the note volume. In the real world, however, most sounds are of decent quality and things like velocity are well-implemented. Because of this, the original goals of the General MIDI System Level 1 specification have largely been realized.
Here are the patch maps for both instruments and drums for Level 1.
GM1 Instrument Patch Map
The General MIDI Level 1 instrument sounds are grouped by families. In each family are 8 specific instruments.
Note: While GM1 does not define the actual characteristics of any sounds, the names in parentheses after each of the synth leads, pads, and sound effects are, in particular, intended only as guides).
Piano Family 1. Acoustic Grand Piano
Chromatic Percussion Family
Synth Lead Family
Guitar Family 25. Acoustic Guitar (nylon) 26. Acoustic Guitar (steel) 27. Electric Guitar (jazz) 28. Electric Guitar (clean) 29. Electric Guitar (muted) 30. Overdriven Guitar31. Distortion Guitar32. Guitar harmonics
Synth Pad Family
Bass Family 33. Acoustic Bass 34. Electric Bass (finger) 35. Electric Bass (pick) 36. Fretless Bass 37. Slap Bass 1 38. Slap Bass 2 39. Synth Bass 1 40. Synth Bass 2
Synth Effects Family
Sound Effects Family
General MIDI Level 1 Percussion Key Map
On MIDI Channel 10, each MIDI Note number corresponds to a different drum sound, as shown below. GM-compatible instruments must have the sounds on the keys shown here. While many current instruments also have additional sounds above or below the range show here, and may even have additional "kits" with variations of these sounds, only these sounds are supported by General MIDI Level 1 devices.